The present invention relates to a parking brake lock-in key switch system for a truck or bus brake system. One type of vehicle on which an embodiment of this invention may be used is a school bus with an air brake system with a driver area operated parking brake actuator. The parking brake actuator is a push pull double check valve. Following a driver manually engaging the parking brake actuator, the parking brake lock-in key switch system will lock the parking brake in the engaged position upon the driver turning the ignition key to the `off` position. This effectively disables the parking brake actuator preventing inadvertent release of the parking brake. This system will not cause an inadvertent initiation of the parking brake should a driver turn the ignition key to the `off` position while the vehicle is moving down the road. The driver must first consciously operate the parking brake actuator for the parking brake lock-in key switch system of this invention to be functional.